diff --git a/backend/src/app.ts b/backend/src/app.ts
index 9881fbe8..b40c915c 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -43,7 +43,10 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
 
   app.addHook("preHandler", authPreHandler(config.authMode));
   app.get("/health", async () => ({ status: "ok" }));
-  sessionRoutes(app, { mgr, tht: tht as ThtRunner, hub, getSettings });
+  sessionRoutes(app, {
+    mgr, tht: tht as ThtRunner, hub, getSettings,
+    ollamaEnsureTimeoutSec: Math.round(config.ollamaEnsureTimeoutMs / 1000),
+  });
   sqlRoutes(app, { tht: tht as ThtRunner });
   metaRoutes(app, { harnessDir: config.harnessDir, listModels });
   settingsRoutes(app, { cfg: config, listModels });
diff --git a/backend/src/config.ts b/backend/src/config.ts
index a10bf53f..d16dd5f8 100644
--- a/backend/src/config.ts
+++ b/backend/src/config.ts
@@ -4,6 +4,7 @@ export interface AppConfig {
   defaults: { provider?: string; model?: string; thinking?: string };
   maxPiProcesses: number;
   settingsFile: string;
+  ollamaEnsureTimeoutMs: number;
 }
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
   return {
@@ -15,5 +16,6 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
     maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
     settingsFile: env.SETTINGS_FILE ?? "data/settings.json",
+    ollamaEnsureTimeoutMs: Number(env.OLLAMA_ENSURE_TIMEOUT_MS ?? 60000),
   };
 }
diff --git a/backend/src/routes/sessions.ts b/backend/src/routes/sessions.ts
index 5afad862..4c21c47d 100644
--- a/backend/src/routes/sessions.ts
+++ b/backend/src/routes/sessions.ts
@@ -7,11 +7,13 @@ import { getUser } from "../auth/auth.js";
 
 export function sessionRoutes(
   app: FastifyInstance,
-  d: { mgr: PiProcessManager; tht: ThtRunner; hub: SseHub; getSettings: () => Settings },
+  d: { mgr: PiProcessManager; tht: ThtRunner; hub: SseHub; getSettings: () => Settings; ollamaEnsureTimeoutSec: number },
 ) {
   app.post("/sessions", async (req, reply) => {
     const b = req.body as { question: string; name?: string };
     const s = d.getSettings();
+    const ensure = await d.tht.ollamaEnsure(s.workspace ?? "", d.ollamaEnsureTimeoutSec);
+    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
     // Settings (global) supply workspace/provider/model/thinking. The new-question
     // form sends only the question text. `workspace` selects the tht `-c <config>`.
     const { id } = await d.tht.sessionNew({
@@ -48,6 +50,8 @@ export function sessionRoutes(
   });
   app.post("/sessions/:id/resume", async (req, reply) => {
     const id = (req.params as any).id;
+    const ensure = await d.tht.ollamaEnsure(d.getSettings().workspace ?? "", d.ollamaEnsureTimeoutSec);
+    if (!ensure.ok) return reply.code(503).send({ error: ensure.error ?? "Ollama/embeddings non disponibili" });
     const manifest = (await d.tht.sessionShow(id)) as { status?: string; archived?: boolean } | null;
     if (manifest?.status === "finalized" || manifest?.archived) {
       return reply.code(409).send({ error: "sessione in sola lettura (finalizzata o archiviata)" });
diff --git a/backend/test/e2e-f1.test.ts b/backend/test/e2e-f1.test.ts
index 8c3f1fb1..2ae45720 100644
--- a/backend/test/e2e-f1.test.ts
+++ b/backend/test/e2e-f1.test.ts
@@ -10,6 +10,7 @@ const SCRIPT = path.resolve("../harness/tests/fake_pi/scripts/f1_disambiguation.
 test("loop F1: crea sessione → SSE riceve il widget → risponde → 204", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
+      ollamaEnsure: async () => ({ ok: true }),
       sessionNew: async () => ({ id: "s1" }),
       sessionShow: async (_id: string) => ({ id: "s1", provider: undefined, model: undefined, thinking: undefined }),
       sessionList: async () => [],
diff --git a/backend/test/routes-sessions.test.ts b/backend/test/routes-sessions.test.ts
index ee01fb00..9162892e 100644
--- a/backend/test/routes-sessions.test.ts
+++ b/backend/test/routes-sessions.test.ts
@@ -9,7 +9,7 @@ const SCRIPT = path.resolve("../harness/tests/fake_pi/scripts/f1_disambiguation.
 
 function mutApp(thtRunner: any) {
   return buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
-    thtRunner,
+    thtRunner: { ollamaEnsure: async () => ({ ok: true }), ...thtRunner },
     getSettings: () => ({ workspace: "w" }) as any,
     spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
   });
@@ -20,6 +20,7 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
   let spawnArg: any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
+      ollamaEnsure: async () => ({ ok: true }),
       sessionNew: async (o: any) => { sessionNewArg = o; return { id: "s1" }; },
       sessionList: async () => [{ id: "s1" }],
     } as any,
@@ -40,6 +41,7 @@ test("POST /sessions usa i settings (workspace/provider/model/thinking) e crea+a
 test("POST /sessions/:id/response inoltra al bridge (no error)", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
+      ollamaEnsure: async () => ({ ok: true }),
       sessionNew: async () => ({ id: "s1" }),
       sessionList: async () => [],
     } as any,
@@ -118,3 +120,47 @@ test("POST resume on an archived session is refused with 409", async () => {
   const res = await app.inject({ method: "POST", url: "/sessions/s1/resume" });
   expect(res.statusCode).toBe(409);
 });
+
+test("POST /sessions refuses with 503 when ollamaEnsure fails (no session created)", async () => {
+  let createdCalled = false;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      ollamaEnsure: async () => ({ ok: false, stage: "model", error: "modello non installato" }),
+      sessionNew: async () => { createdCalled = true; return { id: "s1" }; },
+    } as any,
+    getSettings: () => ({ workspace: "psd" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  expect(res.statusCode).toBe(503);
+  expect(res.json().error).toContain("non installato");
+  expect(createdCalled).toBe(false);
+});
+
+test("POST /sessions proceeds when ollamaEnsure succeeds", async () => {
+  let ensureWs: string | undefined;
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      ollamaEnsure: async (ws: string) => { ensureWs = ws; return { ok: true }; },
+      sessionNew: async () => ({ id: "s1" }),
+    } as any,
+    getSettings: () => ({ workspace: "psd" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  expect(res.json()).toEqual({ id: "s1" });
+  expect(ensureWs).toBe("psd");
+});
+
+test("POST /sessions/:id/resume refuses with 503 when ollamaEnsure fails", async () => {
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: {
+      ollamaEnsure: async () => ({ ok: false, error: "Ollama down" }),
+      sessionShow: async () => ({ status: "open", archived: false }),
+    } as any,
+    getSettings: () => ({ workspace: "psd" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "POST", url: "/sessions/s1/resume" });
+  expect(res.statusCode).toBe(503);
+});